This invention relates to a hologram recording and reproduction apparatus, and more particularly to a hologram recording and reproduction method for recording and reproducing multi-value information into and from a hologram recording medium.
In recent years, increase in recording density of optical disks and so forth has been and is proceeding. However, since the increasing recording density is approaching a physical limit to high density recording, attention is paid to a hologram storage in order to further increase the recording capacity. The hologram storage adopts a recording method of the volume type and allows storage of a greater amount of information when compared with a medium of the planar recording type such as an optical disk. However, if the hologram storage allows multi-value (three- or more-value) recording, then the recording capacity of the hologram storage can be further increased. For such multi-value recording of the hologram storage, multi-value recording depending upon the light intensity is supposed as disclosed, for example, in U.S. Pat. No. 6,512,733.
However, in reproduction of data recorded in a hologram, where the recorded data are, for example, three-value data, a middle gradation must be reproduced. However, the gradation reproducibility is low. According to hologram recording media used at present, an output luminance level characteristic with respect to an input luminance level is represented by a region having a certain extent. This does not decide an output luminance level uniquely with respect to an input luminance level but allows presence of a plurality of output luminance levels. Consequently, different input luminance levels are sometimes represented by the same output luminance level, and in this instance, the gradation reproducibility is low. Further, it is generally difficult to separate different values of data upon decoding of the data upon reproduction, and therefore, reproduction data exhibit a high error rate.